Ultrasonic-driven regulation of solidification dynamics and interfacial integrity in Al/Steel resistance spot welding for enhancing fatigue reliability

Achieving high-cycle fatigue reliability in dissimilar Al/steel joints remains challenging because conventional resistance spot welding (RSW) produces brittle intermetallic compounds (IMCs) and solidification defects under a passive thermal cycle. This work proposes an ultrasonic-assisted resistance spot welding (UA-RSW) process to actively regulate solidification dynamics and interfacial reaction kinetics. The regulatory effects of the ultrasonic energy field on heat and mass transfer, multiscale microstructural evolution, and joint performance are systematically examined. The results demonstrate that the ultrasonic energy field directly alters solidification dynamics. Acoustic streaming and cavitation modify crystallization behavior by fragmenting dendritic arms, promoting a transition from coarse columnar dendrites (57.52 μm) to refined equiaxed grains (12.29 μm). In addition, acoustic streaming-driven convection overcomes viscous drag and establishes a liquid-feeding mechanism to eliminate interfacial shrinkage cavities, while simultaneously modulating reaction kinetics to induce non-equilibrium phase mixing within the nanoscale IMC layer. A non-monotonic relationship between ultrasonic power and joint integrity is identified, with 1200 W representing the optimal process window for achieving maximum cyclic durability. The refined microstructure, featuring a defect-free interface and a mixed-phase IMC layer, promotes a shift in fatigue failure mode from brittle interfacial separation to ductile base-metal fracture. Consequently, the optimized joints show improved cyclic stability, with fatigue life approximately doubling compared with conventional joints at medium load levels. These results confirm the effectiveness of ultrasonic-driven solidification and interface regulation in producing reliable multi-material structures.